Unraveling the Role of Particle Size and Nanostructuring on the Oxygen Evolution Activity of Fe-Doped NiO

Nickel-based oxides and oxyhydroxide catalysts exhibit state-of-the-art activity for the sluggish oxygen evolution reaction (OER) under alkaline conditions. A widely employed strategy to increase the gravimetric activity of the catalyst is to increase the active surface area via nanostructuring or decrease the particle size. However, the fundamental understanding about how tuning these parameters influences the density of oxidized species and their reaction kinetics remains unclear. Here, we use solution combustion synthesis, a low-cost and scalable approach, to synthesize a series of Fe0.1Ni0.9O samples from different precursor salts. Based on the precursor salt, the nanoparticle size can be changed significantly from ∼2.5 to ∼37 nm. The OER activity at pH 13 trends inversely with the particle size. Using operando time-resolved optical spectroscopy, we quantify the density of oxidized species as a function of potential and demonstrate that the OER kinetics exhibits a second-order dependence on the density of these species, suggesting that the OER mechanism relies on O–O coupling between neighboring oxidized species. With the decreasing particle size, the density of species accumulated is found to increase, and their intrinsic reactivity for the OER is found to decrease, attributed to the stronger binding of *O species (i.e., a cathodic shift of species energetics). This signifies that the high apparent OER activity per geometric area of the smaller nanoparticles is driven by their ability to accumulate a larger density of oxidized species. This study not only experimentally disentangles the influence of the density of oxidized species and intrinsic kinetics on the overall rate of the OER but also highlights the importance of tuning these parameters independently to develop more active OER catalysts.


Characterization
Table S1: Results of ICP-OES analysis of doped metal in the NiO in percentage.

Calculation of extinction coefficient
The extinction coefficient was calculated using the stepped potential spectroelectrochemistry method.

Figure S1 :
Figure S1: Cross section SEM images of the as-prepared (A) Acac-(B) Sulfate-(C) Nitrate-and (D) Chloride-derived Fe 0.1 Ni 0.9 O samples.All the samples exhibit similar foamy morphology

Figure S3 :
Figure S3: Selected area electron diffraction (SAED) ring pattern of the NiO sample with overlaid simulated pattern of pure NiO crystal (cubic).

Figure S4 :
Figure S4: (A) STEM image with EDS maps of NiO sample showing the localized presence of (B) Ni (blue) and (C) O (red) and (D) local EDS spectra.

Figure S5 :
Figure S5: TEM images of the Fe 0.1 Ni 0.9 O sulfate sample.

Figure S6 :
Figure S6: (A) HRTEM image of the Fe 0.1 Ni 0.9 O sulfate sample and its (B and C) magnified portions.FFT diffractograms from five different sites with (D) magnified site 5 (representative) showing the fringe-width of 0.2 nm matching with the pure NiO cubic crystal.

Figure S7 :
Figure S7: Selected area electron diffraction (SAED) ring pattern of the Fe 0.1 Ni 0.9 O sulfate sample with overlaid simulated pattern of pure NiO crystal (cubic).Additional reflections indicate the presence of dopants (here, Fe).The occurrence of diffused rings denotes very small particles, resulting in the broadening of peaks in the powder XRD diffractograms as shown in Figure 1E in the main text.

Figure S8 :
Figure S8: (A) STEM image with EDS maps of Fe 0.1 Ni 0.9 O sulfate sample showing the localized presence of (B) Ni (blue), (C Fe (orange), (D) O (red) and (E) local EDS spectra.S trances are detected in the bulk.

Figure S10 :
Figure S10: TEM images of the Fe 0.1 Ni 0.9 O nitrate sample.

Figure S12 :SpectroelectrochemistryFigure S13 :
Figure S12: Cyclic voltammogram of the (A) nitrate -(B) chloride -(C) acac -and (D) sulfatederived Fe 0.1 Ni 0.9 O samples.All measurements were made in 0.1 M Fe-free KOH at a scan rate of 10 mV/s.The lines in darker shades of the corresponding colour indicate the baselines used to determine the area and redox peak centre of the anodic and cathodic redox transitions.

Figure S14 :
Figure S14: Differential UV-vis spectra as a function of potential (noted in the figure) for the (A) nitrate-(B) chloride-(C) acac-and (D) sulfate-derived Fe 0.1 Ni 0.9 O samples.

Figure S15 :
Figure S15: Differential UV-vis spectra as a function of potential (noted in the figure) for the (A) nitrate-(B) chloride-(C) acac-and (D) sulfate-derived Zn 0.1 Ni 0.9 O samples.
A voltage pulse was applied, and the corresponding change in optical absorption and current density were measured simultaneously.The measured optical data is proportional to the density of oxidized species in the sample.Upon switching the potential back to the lower value, a reductive spike in the current is observed, which corresponds to the reduction in the oxidized species.The current-time response during the reductive spike can be integrated to deduce the charge corresponding to reduction in the oxidized states.Using the Lambert-Beer Law, the extinction coefficient can be extracted by plotting the optical signals as a function of the charge.The slope of the graph yields the extinction coefficient.Beer-Lambert Law:  =  *  where: A = absorbance at a given wavelength, ε = extinction coefficient c = concentration of electrons per cm 2

Figure S16 :
Figure S16: Calculation for nitrate-derived Fe 0.1 Ni 0.9 O. (a) electrochemical data and (b) optical data obtained from stepped voltage absorption spectroscopy for increasingly larger potential steps in the OER region.(c) Extinction coefficient obtained from gradient of optical signal to charge.The potential steps were from 1.58 V RHE to 1.66 V RHE .Measurements were made at a wavelength of 750 nm in Fefree 0.1 M KOH.

Figure S17 :
Figure S17: Calculation for chloride-derived Fe 0.1 Ni 0.9 O. (a) electrochemical data and (b) optical data obtained from stepped voltage absorption spectroscopy for increasingly larger potential steps in the OER region.(c) Extinction coefficient obtained from gradient of optical signal to charge.The potential steps were from 1.57 V RHE to 1.63 V RHE .Measurements were made at a wavelength of 500 nm in Fefree 0.1 M KOH.

Figure S18 :
Figure S18: Calculation for acac-derived Fe 0.1 Ni 0.9 O. (a) electrochemical data and (b) optical data obtained from stepped voltage absorption spectroscopy for increasingly larger potential steps in the OER region.(c) Extinction coefficient obtained from gradient of optical signal to charge.The potential steps were from 1.50 V RHE to 1.55 V RHE .Measurements were made at a wavelength of 650 nm in Fefree 0.1 M KOH.

Figure S19 :
Figure S19: Calculation for sulfate-derived Fe 0.1 Ni 0.9 O. (a) electrochemical data and (b) optical data obtained from stepped voltage absorption spectroscopy for increasingly larger potential steps in the OER region.(c) Extinction coefficient obtained from gradient of optical signal to charge.The potential steps were from 1.485 V RHE to 1.51 V RHE .Measurements were made at a wavelength of 850 nm in Fefree 0.1 M KOH.

Figure S20 :
Figure S20: Normalized absorption data for the stepped potential measurements shown in Figure S16-S19 for the (A) nitrate-(wavelength = 750 nm) (B) chloride-(wavelength = 500 nm) (C) acac-(wavelength = 650 nm) and (D) sulfate-(wavelength = 850 nm) derived Fe 0.1 Ni 0.9 O samples.In each panel, a gradient of the corresponding color has been used to represent the data, where the lightest colour represents the lowest potential and the darkest colour of the gradient represents the largest potential step in the measurement.The black curves show the average of all the data sets.

Figure S21 :
Figure S21: Normalized average absorption for the stepped potential measurements for (A) nitrate-(wavelength = 750 nm) (B) chloride-(wavelength = 500 nm) (C) acac-(wavelength = 650 nm) and (D) sulfate-(wavelength = 850 nm) derived Fe 0.1 Ni 0.9 O samples.The points represent the averaged data and the lines represent the best fit based on a single exponential model.The time constant for oxidation or reduction of these species has been annotated.

Figure S22 :
Figure S22: Log-log plot of the current density as a function of the density of oxidized species normalised to the (A) geometric area of the electrode and (B) oxide surface area.

Figure S23 :
Figure S23: Open Circuit decay measurements for the (A) chloride derived sample from 1.57 V RHE (~ 0.06 species/nm 2 oxide ), grey and 1.59 V RHE (~ 0.08 species/nm 2 oxide ), black.The sample is held at constant potential from 10 to 20 seconds followed by open circuit decay from 20 to 100 seconds.Data is reported for a wavelength of 500 nm.(B) acac derived sample from 1.51 V RHE (~ 0.039 species/nm 2 oxide ), light green and 1.53 V RHE (~ 0.05 species/nm 2 oxide ), dark green.The sample is held at constant potential from 20 to 50 seconds followed by open circuit decay from 50 to 100 seconds.Data is reported for a wavelength of 650 nm.